Decreased mucus clearance is a pathologic characteristic of diseases such as cystic fibrosis; chronic obstructive airway disease (also known as chronic obstructive pulmonary disease (COPD)) and asthma. Impaired mucus clearance can also contribute to increased incidence of pulmonary infections and airway obstruction. In particular, cystic fibrosis is characterized by abnormal functioning of the airway epithelial cells. Cystic fibrosis (or “CF”) is caused by a defective gene that codes for a Na+/Cl− transporter present on the surface of the epithelial cells that line the conducting airways of the lung and other organs. Hundreds of mutations have been identified in this gene, all of which result in defective transport of sodium and chloride by epithelial cells. Cystic fibrosis (CF) is the most common autosomal recessive genetic disease in Caucasians causing premature death in the United States. It is caused by mutations in chromosome 7, which code for the cystic fibrosis transmembrane conductance regulator (CFTR). The CFTR encodes for an apical membrane epithelial protein that function as both a cCAMP-regulated chloride channel and a regulator of the epithelial sodium channel. Defects or absence of the CFTR observed in CF patients can be seen as changes in cilia ultrastructure, sodium and chloride ion transport, and water transport across airway epithelial cells. These changes can result in thickened mucous and decreased mucociliary clearance leading to airway infections. This suggests that CFTR in normal lung tissue may regulate the ENaC by down regulating its conductance of sodium ions across the airway epithelium and decreasing water transport into the cell resulting in less viscous mucous and faster mucociliary clearance.
Current treatment for CF has focused on several different therapies but the most effective treatments to date are compounds that change the viscosity of mucous and treat pulmonary infections that arise when bacteria are trapped in the thickened mucous. Therapies to increase mucociliary clearance generally target two different strategies. The first is to regulate sodium absorption into the apical epithelial cells using sodium channel blockers such as amiloride and its derivatives. By decreasing sodium absorption, fluid transport into the epithelial cells is limited and the surface liquid volume is normalized.
The second strategy is to utilize compounds targeting purinergic receptors (i.e. UTP and INS37217) which activate chloride secretion in airway epithelial cells which in turn decreases sodium absorption and increases surface liquid volume. Purinergic receptors are also thought to regulate mucin secretion and to be involved in activation of ciliary beating. By increasing the ciliary beat frequency, mucous transport would be increased which would clear bacteria and other particles from the lungs more rapidly.
These observations indicate that activation of sodium channels can lead to defects in mucus clearance and bronchoconstriction, both of which are associated with airway diseases, including CF.
If activation of voltage gated sodium channels contributes to lung diseases, then effective modulation of voltage gated sodium channels can be useful in alleviating airway pathologies associated with mucociliary dysfunction, such as asthma, chronic obstructive pulmonary diseases, pulmonary infection (e.g., pneumonia, Pseudomonas), and cystic fibrosis. Thus, there is a need for active agents that can modulate water transport across the apical membranes of epithelial cells, these compounds would be are useful in the regulation of mucus clearance, as well as treatment or prevention of conditions or diseases associated with mucociliary dysfunction.
Florida red tides are known to have adverse effects on both marine life and humans. These tides have been linked to large fish kills, marine mammal mortality, and even human illnesses. Human illnesses caused by red tides include respiratory irritation through contact or inhalation and neurotoxic shellfish poisoning (NSP) from consumption of exposed or contaminated seafood (Purkerson-Parker, et al., Chemistry and Biology, 2000, 7: 385–393; Baden, D. G., et al., Toxicon, 1982; 20(5):929–932; Baden, D. G., et al., Int. Rev. Cytol., 1983; 82:99–150). Symptoms of NSP include nausea, vomiting, diarrhea, and bronchoconstriction (Purkerson-Parker, et al. 2000). The causative agent in the red tide organisms has been isolated and identified as brevetoxin.
Ciguatera fish poisoning (CFP) is a form of human poisoning caused by the consumption of subtropical and tropical marine fish that have accumulated naturally occurring toxins through their diet. The toxins are known to originate from several dinoflagellate (algae) species that are common to ciguatera endemic regions in the lower latitudes. Marine fish most commonly implicated in ciguatera fish poisoning include groupers, barracudas, snappers, jacks, mackerel, and triggerfish. Many other species of warm-water fish can harbor ciguatera toxins. The occurrence of toxic fish is sporadic, and not all fish of a given species or from a given locality will be toxic.
Initial signs of poisoning occur within six hours after consumption of toxic fish and typically include a combination of gastrointestinal (e.g., nausea, vomiting, and diarrhea), neurological (e.g., intensified paresthesia, arthralgia, myalgia, headache, temperature sensory reversal and acute sensitivity to temperature extremes, vertigo, and muscular weakness), and cardiovascular disorders (e.g., arrhythmia, bradycardia or tachycardia, and reduced blood pressure). Symptoms defined within these general categories vary with the geographic origin of toxic fish. Diagnosis of CFP remains unsatisfactory and is typically based on patient symptoms and recent dietary history.
Ciguatera poisoning is usually self-limiting, and signs of poisoning often subside within several days from onset. However, in severe cases the neurological symptoms are known to persist from weeks to months. In a few isolated cases neurological symptoms have persisted for several years, and in other cases recovered patients have experienced recurrence of neurological symptoms months to years after recovery. Such relapses are most often associated with changes in dietary habits or with consumption of alcohol. There is a low incidence of death resulting from respiratory and cardiovascular failure.
Current treatments for ciguatera fish poisoning are far from satisfactory. Typically intravenous administration of mannitol is used but is normally only effective if it is used in the first 48–72 hours of exposure. The treatment of chronic CFP is usually symptomatic.
CFP and NSP are thought to be induced via binding at a common receptor site on voltage gated sodium channels known as site 5. Binding by brevetoxins or ciguatoxin at site 5 results in massive influx in sodium ions at normal resting potential.
Thus, there is a need for active agents that can act as antagonists for binding of brevetoxins or ciguatoxin to voltage gated sodium channels, which are useful in alleviating the neurological and gastrointestinal effects in persons affected by NSP and CFP.